The downlink of the 3GPP Long Term Evolution (LTE) cellular communication system is based on Orthogonal Frequency Division Multiplex (OFDM) transmission, which uses time and frequency resource units for transmission. The smallest time-frequency resource unit, called resource element (RE), consists of a single complex sinusoid frequency (sub-carrier) in an OFDM symbol. For the purpose of scheduling transmissions to the different user equipment (UE), the resource elements are grouped into larger units called physical resource blocks (PRBs). A PRB occupies a half of a subframe, called “slot”, consisting of six or seven consecutive OFDM symbol intervals in time domain (0.5 milliseconds in total), and twelve consecutive sub-carrier frequencies in the frequency domain (180 kHz in total).
Downlink (and uplink) transmissions in LTE occur in an orthogonal manner, i.e., in each downlink (/uplink) subframe users are scheduled to receive (/transmit) in orthogonal radio resources by being granted access to non-overlapping sets of physical resource blocks. The downlink of the 3GPP LTE-A system also supports multi-user multiple-input-multiple-output (MU-MIMO) transmission, where a set of users are orthogonally multiplexed in different antenna spatial layers (corresponding to orthogonal radio channels) while being granted access to the same set of time-frequency resources. In this way, more users can be simultaneously served by the base station. Additional non-orthogonal multiple access schemes for LTE-A are currently being investigated by the 3GPP standardization body.
Downlink (/uplink) transmission grants in the LTE system are assigned to the UEs by means of scheduling grants transmitted in one of the downlink control channels provided by LTE: the Physical Downlink Control Channel (PDCCH) and the Enhanced PDCCH (a.k.a. EPDCCH). The PDCCH is transmitted in a control region that can occupy up to three OFDM symbols at the beginning of each subframe, whereas the remaining part of the subframe forms the data region used for the transmission of the Physical Downlink Shared Channel (PDSCH). EPDCCH, on the other hand, consists of a UE-specific control channel configured via higher layer Radio Resource Control (RRC) signalling within the PDSCH resources (i.e., PRB pairs). A UE can be configured to search for UE-specific control information either in the PDCCH region or in the EPDCCH region, while the PDCCH region is monitored to receive control information common to all users connected to a cell.
Both PDCCH and EPDCCH support unicast transmission of control information for various purposes, including dynamic scheduling assignment in downlink and uplink. In addition, PDCCH supports static multicast transmission of common control information, where a plurality of UEs are statically or semi-statically (i.e., for hundreds of transmission time interval (TTIs)) grouped by the network and assigned, for instance, a common Radio Network Temporary Identifier (RNTI) to decode the control information addressed to the group. As the legacy LTE system schedules UEs to access to orthogonal (hence different) radio resources, dynamic multicast transmission of control information to groups of users created on a TTI-basis is not supported.